1. Field of the Invention
The present invention relates to a pressure and heart movement sensor for a heart stimulator, including an electrode system for implantation with an electrode cable containing at least one conductor leading to one electrode pole, or alternately at least two conductors leading to electrode poles.
2. Description of the Prior Art
For arrhythmia treating implants, such as heart stimulators, pressure sensors for controlling the stimulator are ideal sensors in many respects. With a pressure sensor, hemodynamics can be measured in the cases of both tachyarrhythmias and bradyarrhythmias. Heart contractions can be sensed and used for autocapture and atrial synchronization. Furthermore, the time derivative dP/dt is a good rate-response parameter.
For these types of applications, known pressure sensors normally contain a piezoelectric or piezoresistive pressure element. A pacemaker controlled with this type of sensor is previously known from, e.g., U.S. Pat. No. 4,140,132. One disadvantage of this type of sensor is that the sensor requires additional electrical lines, normally two. The electrode cable must therefore contain relatively many conductors. Another disadvantage of this design is that special impedance demands must often be made on the pressure element in order, e.g., to eliminate the problem of leakage resistances, making it necessary to place electric components out near the sensor itself. Another disadvantage is the fact that the piezoelectric ceramic material itself is hard and brittle.
Capacitive accelerometers are also previously known, cf. German OS 3 223 987, in which an inertial mass forms one of the capacitor plates so the capacitance changes when the plate is subjected to acceleration. This technique cannot be used for implantable pressure and heart movement sensors of the type discussed above. Such a sensor would also require additional conductors.
The existence of a capacitive coupling between electrical conductors and between a conductor and ambient medium is well-known, as described by Tietze and Schenk, Halbleiter-Schaltungstechnik, Springer-Verlag, Berlin Heidelberg New York 1978, 4th edition, page 647. The magnitude of the capacitive coupling depends inter alia on the dielectric constant of the materials between the conductors and the distance between them.
Such a capacitive coupling is naturally present even between the conductors in a bipolar pacemaker electrode system and between the conductors and ambient electrolyte, i.e. blood. A capacitance of about 100 pF has been measured between the two conductors in a bipolar pacemaker electrode system in dry conditions. The two conductors here must of course not terminate in electrical poles in direct contact with one another via the electrolyte or body fluid, since this would result in short-circuiting of the capacitance. The resistance between the conductors must be sufficiently large.